18 ga wire: This is my first amp build from the ground up. As such, I used the Hoffman store's 5F6-A parts list to order most of my parts. That parts list includes both 18 and 20 ga wire on the list of parts to build the 5F6-A circuit, so that is what I ordered. In the actual wire/cable section of the parts store, the 18 ga stranded core wire description states: “Use this wire for tube amp switches, filter caps and power section hookups”. That is where I used the 18 AWG wire, based on that description. I had researched all over the internet regarding wire sizes for tube amps, and the answers I found ranged from using anywhere from 18 ga to 24 ga (with multiple conflicting variations as what was acceptable wire size for either power circuits or signal circuits).
Wire voltage rating: I have worked in the aerospace/aviation industries for 39 years, so far (but in mechanical systems, not electronics). In those industries, it is ingrained that you must use materials that are rated for the physical requirements where those materials are used. So, naturally (and apparently in my ignorance) I assumed that a power circuit carrying up to 432 volts in an amplifier would require wire rated in excess of 432 volts (yes, I know it is low amperage).
Sorry to bother y’all with what was apparently a stupid question, but thanks for the answers.